Oil sand extraction processes are used to liberate and separate bitumen from oil sand so that the bitumen can be further processed to produce synthetic crude oil. Numerous oil sand extraction processes have been developed and commercialized, many of which involve the use of water as a processing medium. Other processes are non-aqueous solvent-based processes. Solvent may be used in both aqueous and non-aqueous processes.
One water extraction process is the Clark hot water extraction process (the “Clark Process”). This process typically requires that mined oil sand be conditioned for extraction by being crushed to a desired lump size and then combined with hot (about 95° C.) water and perhaps other agents to form a conditioned slurry of water and crushed oil sand. In the Clark Process, an amount of sodium hydroxide (caustic) is added to the slurry to adjust the slurry pH upwards, which enhances the liberation and separation of bitumen from the oil sand. Other water extraction processes may use other temperatures and may include other conditioning agents, which are added to the oil sand slurry, or may not use a conditioning agent.
Regardless of the type of water extraction process employed, the process will typically result in the production of a bitumen froth that requires treatment with a solvent. For example, in the Clark Process, a bitumen froth stream comprises bitumen, fine particulate solids (also referred to as mineral matter) and water. Certain processes use naphtha to dilute bitumen froth before separating the product bitumen by centrifugation. These processes are called naphtha froth treatment (NFT) processes. Other processes use a paraffinic solvent, and are called paraffinic froth treatment (PFT) processes, to produce pipelineable bitumen with low levels of solids and water. In the PFT process, a paraffinic solvent (for example, a mixture of iso-pentane and n-pentane) is used to dilute the froth before separating the product, diluted bitumen, by gravity. A portion of the asphaltenes in the bitumen is also rejected by design in the PFT process and this rejection is used to achieve reduced solids and water levels. In both the NFT and the PFT processes, the diluted tailings—comprising water, solids and some hydrocarbon—are separated from the product diluted bitumen.
Recovery of solvent from the diluted bitumen component is required before the bitumen may be delivered to a refining facility for further processing. Recovery of the solvent from the diluted tailings component is also desirable for several reasons, since any solvent remaining in the tailings will be discarded with the tailings in a tailings pond. First, a loss of solvent becomes an unnecessary expenditure of the extraction process. Second, any solvent remaining in the tailings pond becomes an environmental issue. Third, water in a tailings pond may be recycled and any solvent remaining in this water may create explosive conditions when reheated for re-use in the various processes.
An example of a PFT process is described further to assist the reader in understanding how the process may be operated. The PFT process may consist of at least three units: Froth Separation Unit (FSU), Solvent Recovery Unit (SRU) and Tailings Solvent Recovery Unit (TSRU). Alternatively, two FSUs may be used.
With reference to FIG. 1, in the FSU unit, mixing of solvent with the feed bitumen froth (100) is carried out counter-currently in two stages with FSU-1 and FSU-2, labeled as Froth Separation Unit 1 (102) and Froth Separation Unit 2 (104). In FSU-1 (102), the froth (100) is mixed with the solvent-rich oil stream (101) from the second stage (FSU-2) (104). The temperature of FSU-1 (102) is maintained at about 70° C. and the target solvent to bitumen ratio (STBR) is about 2:1 (w/w). The overhead from FSU-1 (102) is the diluted bitumen product (105) and the bottom stream from FSU-1 (102) is the tailings (107) consisting of water, solids (inorganics), asphaltenes and some residual bitumen. The residual bitumen from this bottom stream is further extracted in FSU-2 (104) by contacting it with fresh solvent (109) in a 25 to 30:1 (w/w) STBR at about 90° C. The solvent-rich oil (overhead) (101) from FSU-2 (104) is mixed with the fresh froth feed (100) as mentioned above. The bottom stream from FSU-2 (104) is the tailings (111) consisting of solids, water, asphaltenes and residual solvent, which is to be recovered in the Tailings Solvent Recovery Unit (TSRU) (106) prior to the disposal of the tailings (113) in tailings ponds. Solvent from the diluted bitumen overhead stream (105) is recovered in the Solvent Recovery Unit (SRU) (108) and passed as solvent (117) to Solvent Storage (110). Bitumen (115) exiting the SRU (108) is also illustrated.
In the past, STBRs in certain streams have been analyzed by density measurements.